Generally, a shutter and an iris are essential structural elements in an image input device such as a camera. An optimum adjustment of an exposure is the core of subject to be solved in the camera and the exposure depends upon operations of the shutter and the iris. As generally known, the shutter controls duration of the exposure and the iris determines a focusing depth and an opening area. An exposure value is determined by the duration of exposure and the opening area. Further, the focus depth is inversely proportional to the opening area.
The states of the shutter and the iris are determined at user's option in an early type of camera. However, an automatic adjustment art by utilizing electronic elements has been generalized recently.
With reference to FIG. 1, a conventional automatic exposure device includes an exposure sensor 10, a microprocessor 20, a ROM table 30, an iris division 40 and a shutter division 50. When the exposure sensor 10 senses the exposure of an object, the microprocessor 20 controls operations of the iris division 40 and the shutter division 50 according to the data related to a shutter and an iris recorded in the ROM table 30, the data being dependent upon the exposure of the object.
With reference to FIG. 1A, the iris division 40 includes a motor 41 for operating an iris 42 under the control of the microprocessor 20, and a counter 43 for counting the number of revolutions of the motor 41, to thereby calculate the opening area of the iris 42, the information from the counter 43 being transmitted to the microprocessor 20.
With reference to FIG. 11B, the shutter division 50 includes a motor 51 for operating a shutter 52 under the control of the microprocessor 20, and a timer 53 for detecting exposure duration of the shutter 52 to transmit the information to the microprocessor 20.
The microprocessor 20 controls the iris division 40 and the shutter division 50 according to the information recorded in the ROM table 30 and interrupts the operations of the iris 42 and the shutter 52 in case of an operation state of the iris 42 and shutter 52 coinciding with a value predetermined in the ROM table 30.
Depending upon how to determine a correlated adjustment value of the iris 42 and the shutter 52, an automatic exposure device is classified as a program mode, an iris mode, and a shutter mode. In such modes, the microprocessor 20 transmits an exposure value EV detected by the exposure sensor 10 to the ROM table 30 as shown in FIG. 1C.
The ROM table 30 transmits data for establishing operation states of the shutter 42 and the iris 52 according to the exposure value EV to the microprocessor 20. The data for driving the iris 42 is pertaining to the number of revolutions of the motor 41 determining an opening area of the iris 42, and the data for driving the shutter 52 is pertaining to a moving speed V of the shutter 52 and the exposure duration t.
In the iris mode, the moving speed of the shutter V is established after the opening area of the iris is determined. In the meanwhile, in the shutter mode, the opening area of the iris is established after the moving speed of the shutter V is determined. In the program mode, a fine adjustment of the iris 42 and the shutter 52 is required because the operations of the iris and the shutter are correlated with each other.
In such conventional modes, the opening area is fixed during the duration of exposure. A curve shown in FIG. 1D shows the exposure area of the iris with respect to the duration of exposure between the opening and closing of the shutter, in the conventional exposure device including the shutter and the iris being constructed separately.
With reference to FIG. 1D, it is assumed that an opening area A is established as for the effective duration of exposure T.sub.E. The effective duration of exposure T.sub.E can be expressed by an equation T.sub.E =T.sub.2 +(T1+T3)/2=(T0+T2)/2, where T1 is an initial time point of exposure and T3 is a final time point of exposure after opening the shutter. It is regarded that the oblique line T1, T2 for the initial and final time points of exposure is not a straight line in the mathematical sense of the word.
In the iris 42, a scale for regulating an amount of light as the pupil of an eye is indicated as an "F" number. The F number (F#) is determined by a focal length f and an effective diameter D of a lens and can be expressed by a formula F#=f/D (Equation 1). Assuming that a section of exposed area is a circle, the area of exposure A is expressed by a formula A=.pi.R.sup.2 =.pi.D.sup.2 /4 (Equation 2). The exposure value EV of determining an amount of light exposure is defined by a formula 2.sup.ev =F#.sup.2 /T=(ln(.pi.f.sup.2 /4A)-ln Te)/ln 2 (Equation 3). A curve shown in FIG. 1F shows a relation of the effective exposure duration and the exposure value EV drawn by the Equation 3.
In such a conventional automatic exposure device, a mechanism for regulating the iris and the shutter becomes complicated because the iris and the shutter have to be separately included regardless of the operation modes such as the program, the iris, or the shutter modes. In the program mode, it is difficult and complex to control a mechanism interdependently for securing an optimum exposure state, because the shutter and the iris are operating interdependently and a highly precise control is required.
Because locus of the curve at the initial exposure duration T1 and final exposure duration T3 is not a straight line in the mathematical sense of the word, there may occur an error in an amount of exposure expressed as a product of the effective exposure duration Te and the exposure area A of the iris.